Two reproduction effects are involved in a reproduction from a super-resolution optical disk having recording bits equal to or smaller than diffraction limit and recording pits equal to or larger than diffraction limit. One of the effects is an optical signal reproduction effect similar to that of a conventional reproduction scheme for optical disks such as CDs, DVDs, Blu-Ray disks, and HD-DVDs which contribute to the recording pits equal to or larger than diffraction limit. The other is a super-resolution reproduction effect used for reproduction from the recording pits equal to or smaller than diffraction limit. Thus, depending on a pit pattern for recording, the above-described reproduction effects may be synthesized to cause intersymbol interference between the recording pits, which is reflected in the reproduction signal. Then, disadvantageously, determining the symbol of the recording pit pattern based on the reproduction signal becomes difficult, thus reducing bit error rate. Consequently, in order to solve this problem, an array or an arrangement method for recording pit patterns which enables a reduction in the impact of intersymbol interference (for example, Patent Document 1), or for example, a Viterbi determination circuit configured to take the intersymbol interference into account (see Patent Document 2) is used.
However, in the array or arrangement method for recording pit patterns which enables a reduction in intersymbol interference, the effect of the reduction in the intersymbol interference by the arrangement method is limited. Furthermore, in a reduction in intersymbol interference based on the symbols used for recording patterns, recording density may disadvantageously decrease. On the other hand, when the Viterbi determination circuit is used for example that is configured to take into account the intersymbol interference in reproduction signals from a super-resolution optical disk having a mixture of recording pits equal to or smaller than diffraction limit and recording pits equal to or larger than diffraction limit, the number of clocks required for Viterbi determination is equal to time required for a light beam to travel over a distance corresponding to a radius size divided by the period time of one clock. This increases the amount of a determination circuit for the Viterbi determination, thus significantly increasing loads on memories and a CPU for optical disk signal processing. Thus, a signal processing circuit of a large capacity is disadvantageously required.    Patent Document 1: Japanese Patent Laid-Open No. 2007-087494    Patent Document 2: Japanese Patent Laid-Open No. 2005-302095